A raster image processor (RIP) is a unit for interpreting and converting layout information described in a page description language into data information that can be output by an output equipment. For example, an RIP may interpret and convert graphic entities in PDF (Portable Document Format) format into data that can be output by the output equipment. RIP is a core software of the pre-printing industry, which may determine the output speed of a desktop system.
Two imaging models are usually used for drawing page graphic entities by the RIP: substitute imaging model and transparent imaging model. In the substitute imaging model, a graphic entity newly drawn in a page completely substitutes for a background content at a position of the graphic entity, where the background content may be a graphic entity previously drawn in the page. The final colors of points at this position are determined by the graphic entity finally drawn at this position. In the transparent imaging model, a transparent graphic entity newly drawn in a page is subjected to transparent computing with a background content at the position of the graphic entity. That is, the final colors of points at this position are determined by all the graphic entities drawn at the position.
A PDF format is an electronic document format for describing page contents. From version 1.4 of PDF, transparency concept and transparent imaging model are introduced. A PDF page can support various special effects such as transparency, gradient, feathering, and etc.
RIP can interpret and convert the layout information described in a PDF page description language into 1-bit format data or 8-bit format data for output. The 1-bit or 8-bit refers to a number of bits of a color value of a color component of a pixel. For example, if four colors, such as C, M, Y, and K, are used to represent the color of one pixel, when RIP performs 1-bit data output, 4-bit data is required in total, i.e., ½ byte; while when RIP performs 8-bit data output, 32-bit data is required in total, i.e., 4 bytes. If an output equipment requires 1-bit output, under the substitute imaging model, a graphic entity to be drawn is first screened to generate 1-bit data, and the 1-bit data is drawn to an ultimately-output 1-bit page dot matrix. Under the transparent imaging model, if a graphic entity to be drawn is first screened, the resulting 1-bit data cannot be subjected to transparent computing with background colors. Therefore, transparent graphic entities need to be assembled in a manner of 8-bit data output at first, then an 8-bit data dot matrix of the whole layout is screened according to one image-type graphic entity to obtain 1-bit data, and the 1-bit data is drawn to an ultimately-output 1-bit page dot matrix. Therefore, the rasterization speed of the RIP is relatively low.
An early RIP rasterization method comprises determining, by scanning a page, whether transparent graphic entities are contained in the page. If the page contains transparent graphic entities, it is determined to be a transparent page. Each pixel in the transparent page is subjected to transparent mixing operation with the background according to the transparent model, obtaining an ultimate 8-bit page dot matrix. Since not all areas in the transparent page contain transparent graphic entities, such a method may need to handle a large amount of data, resulting in a low rasterization speed.
To improve the rasterization speed of a RIP, in a conventional method, a transparent page is divided into a transparent area containing transparent graphic entities and a nontransparent area not containing transparent graphic entities. The transparent area is assembled according to the transparent model and the nontransparent area is assembled according to the substitute model. The data amount of transparent processing is reduced as compared to the situation in which the entire page is assembled according to the transparent model. However, in the conventional method, assembling the transparent graphic entities of the transparent area is based upon pixels under the equipment coordinate space, and the transparent graphic entities are subjected to transparent computing with background during the assembling. Therefore, when the transparent computing is performed, graphic entity data and background data may already have the equipment resolution. The equipment resolution may be as high as 2,400 dpi, or even higher, thus the data amount of transparent computing may be large. In addition, formula for the transparent computing may also be complex. As a result, the transparent computing may still be time consuming.